It is now common practice that navigation devices (sometimes called a mouse) use coherent light (laser light) to illuminate the surface for navigation purposes. The coherent light reflects from surface imperfections onto a photo-detector array (pixels) thereby providing outputs from the array which are in turn used to determine the direction of movement of the device.
Since coherent light does not degrade with distance, if the light intensity is too great it can damage the human eye. Thus, during manufacture of device using coherent light (such as optical navigation devices) it is necessary to limit the light intensity to a level equal to or less than (≦) a fixed level. This is typically accomplished by limiting power to the light source based upon a light intensity reading. Typically, such calibration is accomplished manually by using a light meter to determine intensity levels and then reducing the power to a point where the intensity is ≦ a particular value. At that point the power level is fixed, usually by reading desired power levels into a register which controls intensity. Once this register is set the power level can not be increased.
It is desirable to reduce manufacturing costs and to improve reliability by using a procedure that is automatic and which does not add complexity to either the manufacturing process or to the circuitry of the navigation device.